Casting is a manufacturing process by which a liquid material is introduced into a mold, which mold contains a hollow cavity of the desired shape, and then allowed to solidify therein. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials, like glass or molten polymer, that solidify or cure after cooling or mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.
The casting sector includes establishments that pour molten ferrous metals (iron and steel) and nonferrous metals under pressure into molds to manufacture castings. Ferrous metal castings include those castings made with grey iron, ductile iron, malleable iron, and steel. Nonferrous castings are predominantly aluminum but also include brass, bronze, zinc, magnesium, and titanium.
The United States is the largest producer of cast products in the world. More than 90 percent of all manufactured goods in the United States contain cast metal components. End-use markets include engine blocks, transmission housings, and suspension parts for cars and trucks; undercarriages for farm and construction equipment; and pipes and valves for plumbing fixtures and boilers.
Jewelry casting is a process of making jewelry pieces that involves the pouring of liquid metal alloy into a mold. The technique has been used for thousands of years, and is still widely used today by both master craftsmen and home crafters to make precise reproductions of original jewelry pieces.
Metal molds or patterns are among the most common in casting processes. Metal patterns are relatively expensive, but are generally dimensionally stable and durable. Metal patterns are used where repetitive production of castings is required in large quantities. However, metal patterns cannot be used when the material being cast has too high a melting temperature. For such applications ceramic molds are used.
Ceramic molds are used to manufacture a variety of useful articles, including metal (for example, titanium) articles such as turbine blades, engine blocks, air intake manifolds, jewelry, etc. While traditional processes such as “lost wax” casting have been utilized to make the molds themselves, more modern approaches including free-form or additive manufacturing by sterolithography with photocurable ceramic resins, has been proposed at least as early as the mid-1990s (see, e.g., J. Halloran et al., U.S. Pat. No. 6,117,612; See also D. Frasier et al., U.S. Pat. Nos. 6,932,145 and 8,550,144; S. Das and J. Halloran, US Patent Application Pub. No. 20100003619; J. Halloran, Stereolithography Apparatus for Free Form Fabrication of Ceramics (Office of Naval Research Final Technical Report, Aug. 25, 1997).
Stereolithography of photocurable ceramic resins may generally be carried out by either a “descending platform” or an “ascending surface” approach. See generally J. Black and R. Kohser, DeGarmo's Materials and Processes in Manufacturing, 514-515 (11th Edition 2011).
In a “descending platform (or “top down”) approach, new layers are formed at the top surface of the growing object. Then, after each irradiation step, the object under construction is lowered into the resin “pool,” a new layer of resin is coated on top, and a new irradiation step takes place. An example of such a technique is given in U.S. Pat. No. 7,892,474. A disadvantage of such techniques is the need to submerge the growing object in a (potentially deep) pool of liquid resin and reconstitute a precise overlayer of liquid resin.
In an “ascending surface” (or “bottom up”) approach, new layers are formed at the bottom of the growing object. Then, after each irradiation step, the object under construction must be separated from the bottom plate in the fabrication well. While such techniques eliminate the need for a deep well in which the object is submerged by instead lifting the object out of a relatively shallow well or pool, a problem with such techniques is that extreme care must be taken, and additional mechanical elements employed, when separating the solidified layer from the bottom plate due to physical and chemical interactions therebetween. For example, in U.S. Pat. No. 7,438,846, an elastic separation layer is used to facilitate separation of solidified material at the bottom construction plane. Other approaches, such as the B9Creator™ 3-dimensional printer marketed by B9Creations of Deadwood, S. Dak., USA, employ a sliding build plate. Such approaches introduce a mechanical step that complicates the apparatus, slows the method, and potentially distorts the end product.
Accordingly, there is a need for alternate methods and apparatus for three-dimensional fabrication of metal casting molds and cores that can obviate the need for mechanical separation steps between the fabrication of successive layers.